Plasma generation is useful in a variety of semiconductor fabrication processes, for example plasma enhanced etching and deposition. Plasmas are generally produced from a low pressure gas by electric field ionization and generation of free electrons which ionize individual gas molecules through the transfer of kinetic energy via individual electron-gas molecule collisions. The electrons are commonly accelerated in an electric field, typically a radio frequency electric field.
Numerous techniques have been proposed to accelerate the electrons in an RF electric field. For example, U.S. Pat. No. 4,948,458 discloses a plasma generating device in which electrons are excited in a radio frequency field within a chamber using a planar antenna coil that is situated parallel to the plane of a semiconductor wafer to be processed. FIG. 1 schematically illustrates a plasma generating device 100 which includes an antenna system 105, a dielectric window 120, a gas distribution plate 130, a wafer to be processed 140, a vacuum chamber 150, an electrostatic chuck 160, and a lower electrode 170.
In operation, a radio frequency source (not shown) is used to provide a radio frequency current to the antenna system 105, typically via a radio frequency matching circuit (also not shown). The radio frequency current resonates through the antenna system 105, inducing an azimuthal electric field within the vacuum chamber 150. At the same time, a process gas is introduced into the vacuum chamber 150 via the gas distribution plate 130, and the induced electric field ionizes the process gas to produce a plasma within the chamber 150. The plasma then impinges upon the wafer 140 (which (which is held in place by way of the electrostatic chuck 160) and processes (e.g., etching) the wafer 140 as desired. Another radio frequency, at a frequency which is different from that applied to the antenna coil, is typically applied to the lower electrode 170 to provide a negative DC bias voltage for ion bombardment.
FIGS. 2A and 2B depict two planar spiral coils 110a, 110b which make up the antenna system illustrated in the '458 patent. As shown in FIG. 2A, a first planar coil 110a is constructed as a singular conductive element formed into a planar spiral and connected to radio frequency taps 205, 215 for connection to radio frequency circuitry. In FIG. 2B, an alternative planar coil 110b is constructed as a plurality of concentric rings 220 connected in series via inter-connectors 225 and coupled at each end to radio frequency taps 205, 215.
As is well known in the art, the circular current pattern provided by such spiral coils creates toroidal-shaped plasmas which can in turn cause radial non-uniformity in the etch rate at the wafer 140. In other words, the E-field inductively generated by the planar coil antenna 110 is generally azimuthal (having a radial component E.sub.r =0 and an azimuthal component E.sub..theta. .noteq.0), but zero at the center (E.sub.r =0 and E.sub..theta. =0). Thus, the coil antenna 110 produces a toroidal plasma having a lower density in the center, and must rely on plasma diffusion (i.e., the diffusion of electrons and ions into the center) in order to provide reasonable uniformity at the center of the toroid. In certain applications, however, the uniformity provided by plasma diffusion is insufficient.
Further, such spiral coil antennas tend to make azimuthal non-uniform plasma. This results from the fact that the relatively long lengths of coupling lines used to construct the planar antenna coils have significant electrical lengths at the radio frequency at which they typically operate. The voltage and current waves travel forward from the input end to the terminal end, and will be reflected back at the terminal end. The superposition of the forward and reflected waves results in a standing wave on the coil (i.e., the voltage and current vary periodically along the length of the coil). If the coil is grounded at the terminal end, the current at the terminal end is at a maximum value, and the voltage at the terminal end is zero. Proceeding along the coil toward the input, the voltage increases and the current decreases until, at 90 degrees of electrical length, the voltage is at a maximum and the current is at a minimum. Such a degree of variation results in a highly non-uniform plasma. Consequently, the planar coil is typically terminated with a capacitance such that the current in the coil is similar at both ends of the coil and increases to a maximum near the middle of the coil. Doing so can improve plasma uniformity, but azimuthal non-uniformity still exists because the current varies in the azimuthal direction along the length of the coil. For example, point P in FIG. 2A is the current maximum. On either side of point P the current drops off. Therefore, the power coupling to the plasma is higher beneath P and the corresponding plasma is denser. In contrast, the plasma density at point P' is relatively lower.
Note that, although the terminating capacitor value can be varied, doing so only changes the position of the voltage null along the coil. Further, although the coil can be terminated with an inductance in order to provide the same polarity voltage along the coil length, a current null will exist somewhere in the central portion of the coil (with the current traveling in opposite directions on either side of the null), and the resulting plasma density can be unacceptably low and non-uniform. U.S. Pat. No. 5,401,350 to Patrick et al. attempts to overcome the above-described deficiencies. Therein, a multiple planar coil configuration is set forth in order to improve plasma uniformity. The RF power to the individual coils is independently controlled, requiring separate power supplies and separate matching networks which allow for independent adjustment of the power and phase.
It is evident that a need exists for improved methods and apparatuses for controlling the inductive coupling uniformity within a plasma coupled system.